Preparation method of flocculant for flue gas desulfurization wastewater from catalytic cracking

　　In the field of petroleum refining,the treatment of catalytic cracking flue gas desulfurization wastewater has always been a critical and challenging step.This type of wastewater has complex components and contains a large amount of suspended solids,heavy metal ions,and high concentrations of harmful substances such as chloride ions.If discharged directly without effective treatment,it will cause serious pollution to the environment.As the core agent for wastewater treatment,the preparation method of flocculant for flue gas desulfurization wastewater from catalytic crackings directly affects the treatment effect and cost.This article will delve into the preparation methods of coagulants,aiming to provide scientific basis and technical support for practical industrial applications.

1.Classification and mechanism of action of coagulants

　　1.1 Charge neutralization and adsorption bridging of inorganic flocculant for flue gas desulfurization wastewater from catalytic crackings
　　Inorganic coagulants are represented by aluminum salts and iron salts,such as polyaluminum chloride(PAC),polyferric sulfate(PFS),etc.Its mechanism of action is based on double layer compression and charge neutralization:when the pH value of the wastewater is between 5.0-9.0,Al³⁺in PAC hydrolyzes to form Al(OH)∝colloids,which adsorb negatively charged colloidal particles in the wastewater to form large particle floc precipitation.Experiments have shown that PAC can achieve a removal rate of over 90%for particles with a diameter greater than 10μm in catalytic cracking wastewater.However,when used alone,there are problems such as insufficient floc density and slow settling speed.
　　1.2 Molecular chain bridging and net trapping of organic flocculants
　　Organic flocculant for flue gas desulfurization wastewater from catalytic crackings are mainly composed of polyacrylamide(PAM)and its derivatives.The amide groups(-CONH₂)on their molecular chains can adsorb suspended solids in wastewater through hydrogen bonding,while the long-chain structure forms a"net trapping"effect,encapsulating small particles into large flocs.Cationic PAM(CPAM)can undergo electrostatic neutralization reaction with negatively charged colloidal particles due to its positive charge,further enhancing the flocculation effect.Research has shown that CPAM has a 25%higher removal rate than PAC for particles with a diameter of less than 5μm in catalytic cracking wastewater.However,when used alone,it has drawbacks such as slow dissolution rate and susceptibility to moisture absorption and agglomeration.
　　1.3 Synergistic mechanism of inorganic organic composite flocculants

　　In response to the complex composition of catalytic cracking wastewater,inorganic organic composite flocculant for flue gas desulfurization wastewater from catalytic crackings achieve efficient flocculation through a multi-stage mechanism of"charge neutralization bridging adsorption net trapping and sweeping".For example,when PAC is combined with CPAM,PAC first stabilizes the colloid through charge neutralization,and CPAM then aggregates the stabilized particles into large flocs through molecular chain bridging,ultimately achieving solid-liquid separation through net trapping.Experimental data shows that the PAC+CPAM composite flocculant can achieve a suspended solids removal rate of over 99%in catalytic cracking wastewater,which is 15%-20%higher than that of a single flocculant,and the settling speed of flocs is increased by 30%.

2.Preparation process of flocculant for flue gas desulfurization wastewater from catalytic cracking

　　2.1 Raw material selection and proportioning design:precise control based on wastewater characteristics
　　The composition of catalytic cracking wastewater is complex,including oil,catalyst dust,sulfides,and heavy metal ions.The flocculant formula needs to be designed according to the water quality characteristics.For example:
　　High suspended solids wastewater:PAC as the main agent,with a ratio of 150-300mg/L,supplemented by 5-10mg/L CPAM,can quickly form large flocs;
　　High COD wastewater:Adding chelating agents such as hydroxyethyl diphosphonic acid(HEDP)in combination with PAC can remove more than 80%of organic matter in the wastewater;
　　High salinity wastewater:Salt tolerant flocculant for flue gas desulfurization wastewater from catalytic crackings such as JQL01 chitosan quaternary ammonium salt are used,which maintain a flocculation efficiency of over 90%even when the Cl⁻concentration is greater than 5000mg/L.
　　2.2 Preparation process:from solution preparation to performance testing
　　2.2.1 Preparation of Inorganic coagulants
　　Taking PAC as an example,its preparation process includes three steps:acid dissolution of calcium aluminate powder,hydrolysis polymerization,and maturation:
　　Acid dissolution:Mix calcium aluminate powder with hydrochloric acid in a mass ratio of 1:3,stir and react at 80-90℃for 2 hours to generate AlCl3 solution;
　　Hydrolytic polymerization:Slowly add NaOH solution dropwise to AlCl∝solution,adjust the pH to 4.5-5.0,and promote the hydrolysis of Al³⁺to form Al(OH)∝colloids;
　　Maturation:Let it stand at 60-70℃for 24 hours to further polymerize the colloidal particles and form a high degree of polymerization PAC solution.
　　2.2.2 Preparation of organic flocculant for flue gas desulfurization wastewater from catalytic crackings
　　Taking CPAM as an example,its preparation process includes three steps:acrylamide polymerization,cation modification,and drying and crushing:
　　Aggregation:Mix acrylamide(AM)with methacryloyloxyethyltrimethylammonium chloride(DMC)in a 9:1 mass ratio,add initiator ammonium persulfate(APS),and polymerize at 40-50℃for 4 hours to form a copolymer;
　　Modification:Add NaOH solution to the copolymer to adjust the pH to 9.0-10.0,react at 60-70℃for 2 hours,and introduce cationic groups;
　　Drying and crushing:the modified polymer is dried and crushed by spray to obtain CPAM powder with particle size<100μm.
　　2.2.3 Preparation of composite flocculants
　　Mix the prepared PAC and CPAM in a mass ratio of 10:1-20:1,and stir at a speed of 500-800r/min for 30 minutes in a high-speed mixer to fully mix the two flocculants and form a uniform composite solution.During the mixing process,the temperature should be controlled to be≤40℃to avoid performance degradation caused by CPAM molecular chain breakage.
　　2.3 Performance Testing and Optimization:A Bridge from Laboratory to Industrial Applications
　　The performance testing of flocculants needs to simulate actual wastewater treatment conditions,with a focus on testing the following indicators:
　　Flocculation efficiency:Measure the turbidity changes before and after wastewater treatment using a turbidity meter,and calculate the removal rate;
　　Flock settling velocity:Record the time required for flocs to form and fully settle,and evaluate the improvement effect of flocculant for flue gas desulfurization wastewater from catalytic crackings on settling performance;
　　Temperature and acid alkali resistance:Test the stability of flocculant performance under conditions of 40-80℃and pH 5.0-9.0;
　　Residual detection:The residual amount of metal ions such as Al and Fe in the treated wastewater is detected by atomic absorption spectroscopy(AAS)to ensure compliance with discharge standards.

　　According to the test results,the performance of flocculant for flue gas desulfurization wastewater from catalytic crackings can be further improved by adjusting the raw material ratio and optimizing the preparation process parameters(such as reaction temperature,time,pH value).For example,reducing the polymerization temperature of PAC from 70℃to 60℃can reduce the excessive polymerization of Al(OH)3 colloidal particles and improve the capturing ability of flocculants for small particles.

3.Optimization and evaluation of flocculant performance

　　3.1 Key Performance Indicators
　　Flocculation efficiency:suspended solids removal rate≥99%,effluent turbidity≤50NTU;
　　Temperature resistance:Maintain activity within the range of 40-80℃,with floc settling rate≥5cm/min;
　　Impact resistance:When the concentration of suspended solids fluctuates by±2000mg/L,the flocculation effect remains stable;
　　Environmental friendliness:residual aluminum ion concentration≤0.2mg/L,in compliance with the Comprehensive Wastewater Discharge Standard(GB 8978-1996).
　　3.2 Direction of Process Optimization
　　PH adjustment:By adding lime or sulfuric acid,the pH of the wastewater is controlled at 6.5-7.5 to improve the charge neutralization efficiency of the flocculant for flue gas desulfurization wastewater from catalytic cracking;
　　Addition method:Multiple point addition(such as slurry pool,reaction tank inlet)is used to extend the contact time between the flocculant and particles;

　　Collaborative processing:Used in combination with oxidants(such as sodium hypochlorite)to simultaneously reduce COD and chromaticity.

　　In summary,the preparation method of the flocculant for flue gas desulfurization wastewater from catalytic cracking introduced in this article has successfully produced an efficient,economical,and environmentally friendly flocculant through reasonable selection of raw materials and optimization of process conditions.This flocculant has demonstrated excellent performance in treating catalytic cracking flue gas desulfurization wastewater,effectively removing suspended solids and heavy metal ions from the wastewater,reducing equipment corrosion risks,and lowering treatment costs.In the future,we will continue to optimize and improve this preparation method,promote its widespread application in industry,and contribute to the green development of the refining industry.